Severe Thunderstorm Warning 379, NWS Lincoln, IL

Area Forecast Discussion
National Weather Service Lincoln IL
550 PM CDT Sun Sep 20 2026

.KEY MESSAGES...

- A Marginal Risk (level 1 of 5) of severe weather exists near
  and south of the I-72 corridor today. The primary risk will
  be isolated strong wind gusts during the afternoon and early
  evening.

- Much cooler weather will be on tap this week as high
  temperatures drop into the 60s everywhere north of I-70
  Monday through Wednesday.

&&

.DISCUSSION...
Issued at 215 PM CDT Sun Sep 20 2026

NEAR TERM /THROUGH TONIGHT/

Latest surface observation network and mesonet analysis show an
effective surface boundary positioned along the I-70 corridor
early this afternoon. A distinct thermal gradient exists across
the front, with temperatures north of the boundary holding in
the upper 70s to near 80 F, while warm sector air to the south
has warmed into the mid to upper 80s. Moisture profiles remain
relatively uniform across the zone, with little dewpoint air-
mass contrast observed.

Regional radar mosaic reveals widely scattered showers
continuing near and just north of the surface boundary, driven
primarily by gentle isentropic ascent overriding the frontal
slope. Visible satellite imagery shows an agitated cumulus field
expanding near a differential heating boundary. Peaks of
sunshine are generating modest destabilization this afternoon,
with MLCAPE values climbing around 1500 J/kg. This instability
overlaps a favorable kinematic environment featuring roughly 35
kt of 0-6 km bulk shear, enhanced by a 500 mb speed maximum
rounding the larger synoptic trough axis overhead.

This parameter space supports isolated organized updrafts
capable of producing a few marginally severe storms through the
late afternoon. High-resolution CAM guidance (including recent
runs of the HRRR and RRFS) continues to hint at isolated
convective development north of the effective boundary likely
focused along a local differential heating boundary and
triggered by subtle mid-level shortwaves. Trends on satellite
and radar will be monitored closely for convective initiation
across central and southeastern portions of the forecast area.

By tonight, precipitation coverage is expected to expand along
and north of the surface front. Strengthening low-level jet
(LLJ) convergence and additional shortwave impulse arrivals will
impinge on the sharpening baroclinic zone. Guidance points to
the highest coverage of showers and embedded convection
targeting areas south of I-72 through early Monday morning.

SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/

Rain showers will gradually diminish from west to east Monday
morning as the low-level jet recedes, and forcing from the
departing shortwave pushes toward the upper Ohio Valley.

In the wake of this wave, a cooler and drier air mass will
build across central Illinois as high pressure settles beneath a
broader split-flow blocking regime aloft. However, a cutoff or
weak upper-level low meandering within the split flow across the
Mid-Mississippi Valley will keep weather conditions slightly
unsettled. Daily forecast trends through Wednesday afternoon
will need to account for periods of cloudiness, low-end
shower/sprinkle chances, and potentially localized cold-air
funnel activity given the cold pocket aloft and light surface
winds under the upper trough. High temperatures throughout the
early to mid-week period will remain seasonably cool, topping
out near or just below 70F with overnight lows falling into the
low to mid-50s.

LONG TERM /THURSDAY THROUGH SUNDAY/

The stubborn upper-level trough finally kicks eastward by
Thursday, allowing a broader upper-level ridge to build across
the Midwest and Great Lakes region. Upper-level heights will
rise through late week, effectively suppressing lingering
precipitation potential and yielding sustained fair weather into
the weekend. Temperatures will gradually moderate under the
building ridge, keeping conditions comfortable with highs near
seasonal averages.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 550 PM CDT Sun Sep 20 2026

IFR ceilings have overspread KPIA-KBMI late this afternoon, and
will continue expanding south to affect KCMI-KDEC-KSPI over the
next few hours. These ceilings should persist through much of
Monday, with a 60-80% chance they rise to low MVFR category
after 18z. After a lull in precip for much of the evening,
another wave will spread northeast after 05z, with short range
guidance showing this reaching as far north as the I-72
corridor. Went with PROB30 TSRA groups for KSPI-KDEC-KCMI
overnight, and will monitor trends as this develops. Northeast
winds at 08-14 kt will be persistent through the forecast.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.

&&

KEY MESSAGES...MJA
DISCUSSION...MJA
AVIATION...25

Hazardous Weather Outlook
National Weather Service Lincoln IL
252 PM CDT Sun Sep 20 2026

ILZ027>031-036>038-040>057-061>063-066>068-071>073-211200-
Knox-Stark-Peoria-Marshall-Woodford-Fulton-Tazewell-McLean-Schuyler-
Mason-Logan-De Witt-Piatt-Champaign-Vermilion-Cass-Menard-Scott-
Morgan-Sangamon-Christian-Macon-Moultrie-Douglas-Coles-Edgar-Shelby-
Cumberland-Clark-Effingham-Jasper-Crawford-Clay-Richland-Lawrence-
252 PM CDT Sun Sep 20 2026

This Hazardous Weather Outlook is for central Illinois, east
central Illinois, southeast Illinois and west central Illinois.

.DAY ONE...Tonight.

Periodic showers and thunderstorms are expected across central
Illinois late this afternoon and tonight. Some of the storms could
produce locally heavy rain that results in flooding of low lying and
poor drainage areas. A few strong to severe storms are possible
mainly along and south of I-72/Danville. Damaging wind gusts will be
the primary hazard.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed later this afternoon and evening.

Expected storm motion is toward the east around 30 mph.

Auten